According to LTE (Long Term Evolution) that is being standardized in 3GPP (3rd Creation Partnership Projects) at present, there has been proposed a wireless communication system which includes EUTRAN (Evolved UMTS Terrestrial Radio Access Network, UMTS=Universal Mobile Telecommunication System) and EPC (Evolved Packet Core) that are configured as shown in FIG. 1 (4.2.1 of Non-patent document 1, FIGS. 4.2.1-1 and 4.2.1-2 of Non-patent document 2). The above titles are not restrictive, but EUTRAN may be called “LTE”, EPC may be called SAE (System Architecture Evolution), and EUTRAN and EPC may collectively be called EPS (Evolved Packet System).
As shown in FIG. 1, the EUTRAN includes eNode B (evolved Node B) 10 as a base station. The EPC includes CN (Core Network) Nodes comprising MME (Mobility Management Entity) 20 as a mobility management node, S-GW (Serving Gateway) 30 as a gateway, P-GW (Packet Data Network Gateway) 40 as a higher-level gateway, and HSS (Home Subscriber Server) 50. eNode 10 is connected to UE (User Equipment) as a wireless communication apparatus through a wireless interface.
MME 20 is a node having a mobility management (location registration) function for UE 60, a handover control function, a selection function for S-GW 30 and P-GW 40, a bearer management function, etc (4.4.2 of Non-patent document 1). S-GW 30 is a node for transferring user-plane packet data between eNode B 10 and P-GW 40. P-GW 40 is a node for transferring transmission packet data from its own network (Home PLMN, PLMN=Public Land Mobile Network) to an external network (Visit PLMN) and transferring reception packet data from an external network to its own network. HSS 50 is a server for saving user information that is used to authenticate UE 60.
According to LTE, TAs (Tracking areas) are assigned to UE 60 as areas in which UE 60 is to be paged when an incoming call is received (5.2.3 of Non-patent document 1). Specifically, when UE 60 registers its location in eNode B 10, TAs are assigned to UE 60 by MME 20, and the list of assigned TAs is registered in UE 60. If UE 60 detects when it has moved to a TA that is not included in the registered list, UE 60 registers its location again in eNode B 10 in order to update the TAs (5.3.3.1 of Non-patent document 1).
In a region where the paging traffic is high, the number of TAs that are assigned to UE 60 when UE 60 registers its location is reduced in order to reduce the number of areas in which UE 60 is to be paged. In order to reduce the number of times that UE 60 registers its location, on the other hand, the number of TAs assigned to UE 60 which is moving at a high speed is increased.
Consequently, there is a trade-off between the number of TAs for reducing the paging traffic and the number of TAs for reducing the number of times that UE 60 registers its location. It is thus necessary to optimize the number of TAs assigned to UE 60 in view of the trade-off.
A general process of assigning TAs to UE 60 will be described below.
It is assumed that from among the respective cells of a plurality of eNodes B 10, cells C#1 through C#23 are arranged as shown in FIG. 2, and cells C#1 through C#23 belong to TA#1 through TA#7 as follows:
TA#1=C#1, C#2, C#3, C#4, C#5
TA#2=C#17, C#18, C#19, C#20, C#21, C#22
TA#3=C#6, C#7, C#8
TA#4=C#9, C#10, C#12, C#13, C#14
TA#5=C#16
TA#6=C#11, C#15
TA#7=C#23
Generally, MME 20 assigns TAs to UE 60 according to a rule that is manually established by the operator. According to the rule, a plurality of TAs are fixedly assigned to UE 60.
Specifically, in the example shown in FIG. 2, the rule is such that when UE 60 registers its location in either one of eNodes B 10 of the cells belonging to TA#1, two TAs represented as TA#1 and TA#4 are fixedly assigned to UE 60.
Even when UE 60, which is moving at a high speed, registers its location in either one of eNodes B 10 of the cells belonging to TA#1, TA#1 and TA#4 are assigned to UE 60.
If the cells belonging to TA#1 and TA#4 are of the type which covers a very small range (having a radius of several hundreds meters), for example, then even though MME 20 assigns TA#1 and TA#4 to UE 60, since UE 60 travels through TA#1 and TA#4 and enters TA#5 in several seconds, UE 60 needs to newly register its location.
The time which UE 60 takes to travel through TA#1 and TA#4 will be actually calculated as described below.
It is assumed that UE 60 registers its location in eNode B 10 of C#2 belonging to TA#1, the cells belonging to TA#1 and TA#4 have a diameter of 500 m (meter), and UE 60 travels at a speed of 80 km (kilo meter)/h (hour).
The distance over which UE 60 travels through TA#1 and TA#4 is 2500 m (meter) across five cells (C#2, C#3, C#5, C#12, C#13) (=500 m*5).
Therefore, the time which UE 60 takes to travel through TA#1 and TA#4 is 113 seconds (≈2500 m/80 km/h). This numerical value indicates that UE 60 will do its location registration in about two minutes. Therefore, the number of times that UE 60 registers its location cannot be reduced.
Since the number of times that UE 60 registers its location cannot be reduced, an optimum number of TAs cannot be assigned to UE 60.
According to the practice of fixedly assigning a plurality of TAs to UE 60, a plurality of TAs are also assigned to UE 60 which mostly does not move in daytime. Consequently, since the paging traffic for paging UE 60, when an incoming call is received, has to cover the plural TAs, the paging traffic cannot be reduced, resulting in a large burden imposed on the wireless communication system.
As described above, the practice of fixedly assigning a plurality of TAs to UE 60 is problematic in that an optimum number of TAs cannot be assigned to UE 60.
Furthermore, in as much as the operator manually sets the rule for assigning TAs to UE 60 in MME 20, the rule has to be re-established each time eNode B 10 is added or removed. This requires the operator to spend a lot of time and make a lot of effort, and hence results in an increase in OPEX (Operation and Expenditure).    Non-patent document 1: 3GPP TS 23.401, V8.0.0    Non-patent document 2: 3GPP TS 36.300, V8.2.0